An increasing customer demand for data services, wherein communication terminals, in particular mobile phones, are used as data terminal, induce the operators of communication networks to offer their customers internet accesses or own data services by means of so-called application servers, which are attached to their own network operating packet data network (IP), more frequently. Such application servers provide, for example, current news, soccer results, traffic information or market rates. Examples for such services are the IMODE™ service from NTT Docomo, the Sky Walker™ or the Sky Web™ service from the Digital Phone Group.
There are communication terminals such as mobile phones, which are sold by the data service provider and which offer such a special data service. For the call-up thereof said devices often have a simple user interface, such as a separate key on the keyboard, by means of which such a service, e.g. a traffic information service can be requested. If a user, therefore, requests the traffic information service by pressing a key, the mobile phone sends a service request. The addressee of said service request is a corresponding application server, which is identified by its so-called Internet protocol address, hereinafter called IP address. The IP address of said application server, the data service of which can be called up by the mobile phone, is therefore unchangeably stored in a read-only memory of the device. Alternatively, the IP address can also be entered and stored by the user.
An increasing demand for the data service can exhaust the processing capacity of the application server. In this case, the service provider will install an additional application server for this data service in order to satisfy the demand. Said additional application server is identified by another IP address, as different physical servers should not have the same IP address.
In order to allow an additionally installed application server to successfully function as a load distribution measure, a change in the hardware, i.e. the exchange of the corresponding memory chip is required for part of the circulating communication terminals with fixedly stored IP addresses. Alternatively, the production can be changed, so that as a new edition only terminals are manufactured, which contain the IP address of the additional application server. Both alternatives are not very flexible and involve expenses and partly highly logistic labor. The same refers to terminals in case of which the IP address is entered and stored by the user. Beside the work to inform part of the users about the changed IP address, there is an insecurity with regard to the efficiency of the load distribution measure. The respective user is not obliged to change the IP address, as he will still be reaching an application server under the previously known IP address, which offers the desired data service.
A known solution for distributing the load of the data traffic to several servers is offered by the so-called LSNAT method, which is described in the RFC 2391 “Load Sharing using Ip Network Address Translation (LSNAT)” by P. Srisuresh and D. Gan and which was published by the IETF. In this method packet data flows directed to a single IP address of a server are distributed to a pool of servers. The distribution takes place session-oriented by a so-called LSNAT router with the aid of real time load distribution algorithms, such as the so-called Least Load First Algorithm or the so-called Weighted Least Traffic First Algorithm. For this purpose a network address translator NAT translates the IP addresses of the datagrams, i.e. of the data packets containing address and route information.
For using said load distribution method special routers, so-called LSNAT routers are required in the communication network, which causes additional costs. Furthermore, it has to be secured that all data packets of a session are forwarded by the same LSNAT router, which can imply considerable work depending on the existing network architecture. Finally, the applied load distribution algorithm can cause a bottleneck of the system, particularly in systems comprising a large number of servers.
One alternative is the use of symbolic address identifiers for application servers, such as “www.anwendungsserver.de”, instead of the IP address, for example “3.1.226.64”. These symbolic address identifiers have to be resolved by a so-called DNS server request before calling up the data service. By considering the load situation of the server of the communication system, the DNS server can undertake a load distribution when the symbolic address identifiers are resolved. The system bottleneck is, however, not removed by this solution, but only shifted from the application server to the DNS server. In this case, too, for calling up the DNS server, the terminal requires the IP address thereof. In the case of a high demand for data services the number of DNS server requests augments so that the DNS server can be overloaded.
As was explained above, the capacity enlargement of server-oriented data service requires the addition of more servers. Due to the new IP addresses of said servers, which are thereby inevitably introduced, a measure for distributing the load is necessary. Known methods for load distribution require additional work by the service provider or the network operator, such as the application of special routers on the network side, or the modification of the terminal hardware by the manufacturer or, respectively, the modification of the terminal software by the user.